Britannia is not Britain. If you've seen the map of Britannia (and the world) in Code Geass, it doesn't even include the British Isles. The homeland of Britannia is what is geographically in the same area as the United States of America.

Britannia is not Britain. If you've seen the map of Britannia (and the world) in Code Geass, it doesn't even include the British Isles. The homeland of Britannia is what is geographically in the same area as the United States of America.

Britannia is not Britain. If you've seen the map of Britannia (and the world) in Code Geass, it doesn't even include the British Isles. The homeland of Britannia is what is geographically in the same area as the United States of America.

Britannia is not Britain. If you've seen the map of Britannia (and the world) in Code Geass, it doesn't even include the British Isles. The homeland of Britannia is what is geographically in the same area as the United States of America.

The only part I can vouch for, however, based on information in the series is the part about Britain being conquered early on, and the monarchy moving to what is the current homeland (i.e., what we know as the United States). That much was mentioned in passing in the series.

I suspect Code Geass is the only anime series to date where Britain and Britannia aren't the same geographically (as in Strike Witches, for example), so it is a understandable mistake.

That said, technically anyone from a country called Britannia, regardless of location would technically be Britannian, not British.